Do not clear cache in __del__

Prior to this change, running
`py.test --assert=rewrite test/test_integration.py` fails with errors
due to:

```py
cls = <class 'jedi.recursion.ExecutionRecursionDecorator'>

    @classmethod
    def cleanup(cls):
>       cls.parent_execution_funcs.pop()
E       IndexError: pop from empty list

recursion.py:127: IndexError
```

Similar errors occurred in travis occasionally:
- https://travis-ci.org/davidhalter/jedi/jobs/5449831
- https://travis-ci.org/davidhalter/jedi/jobs/5512538

I think this is because GC calls __del__ at random point during
actual execution of ExecutionRecursionDecorator.__call__.
As --assert=rewrite works by AST dynamically, I guess that it
is harder for Python's GC to clean up code and therefore GC
happens sometime later.  Although this is just a random guess,
as `tox -- --assert=rewrite` works with this patch now, I think
this is a good change.

jedi/api.py

@@ -76,6 +76,7 @@ class Script(object):
         """ lazy parser."""
         return self._module.parser
 
+    @api_classes._clear_caches_after_call
     def complete(self):
         """
         Return :class:`api_classes.Completion` objects. Those objects contain
@@ -209,6 +210,7 @@ class Script(object):
         warnings.warn("Use line instead.", DeprecationWarning)
         return self.definition()
 
+    @api_classes._clear_caches_after_call
     def definition(self):
         """
         Return the definitions of a the path under the cursor. This is not a
@@ -272,6 +274,7 @@ class Script(object):
                     if not isinstance(s, imports.ImportPath._GlobalNamespace)])
         return sorted(d, key=lambda x: (x.module_path, x.start_pos))
 
+    @api_classes._clear_caches_after_call
     def goto(self):
         """
         Return the first definition found by goto. Imports and statements
@@ -334,6 +337,7 @@ class Script(object):
                     definitions = [user_stmt]
         return definitions, search_name
 
+    @api_classes._clear_caches_after_call
     def related_names(self, additional_module_paths=()):
         """
         Return :class:`api_classes.RelatedName` objects, which contain all
@@ -378,6 +382,7 @@ class Script(object):
         warnings.warn("Use line instead.", DeprecationWarning)
         return self.function_definition()
 
+    @api_classes._clear_caches_after_call
     def function_definition(self):
         """
         Return the function object of the call you're currently in.
@@ -487,9 +492,6 @@ class Script(object):
         match = re.match(r'^(.*?)(\.|)(\w?[\w\d]*)$', path, flags=re.S)
         return match.groups()
 
-    def __del__(self):
-        api_classes._clear_caches()
-
 
 def defined_names(source, source_path=None, source_encoding='utf-8'):
     """

jedi/api_classes.py

@@ -7,6 +7,7 @@ interesting information about completion and goto operations.
 import re
 import os
 import warnings
+import functools
 
 from _compatibility import unicode, next
 import cache
@@ -34,6 +35,18 @@ def _clear_caches():
     imports.imports_processed = 0
 
 
+def _clear_caches_after_call(func):
+    """
+    Clear caches just before returning a value.
+    """
+    @functools.wraps(func)
+    def wrapper(*args, **kwds):
+        result = func(*args, **kwds)
+        _clear_caches()
+        return result
+    return wrapper
+
+
 class BaseDefinition(object):
     _mapping = {'posixpath': 'os.path',
                'riscospath': 'os.path',