Add neovim 0.5 to list of test runtimes. (#3830)

* Add neovim 0.5 to list of test runtimes.

* Update testbed docker image

Co-authored-by: Horacio Sanson <horacio@allm.inc>

Dockerfile

@@ -1,15 +1,17 @@
-FROM tweekmonster/vim-testbed:latest
+FROM testbed/vim:20
 
 RUN install_vim -tag v8.0.0027 -build \
                 -tag v8.2.2401 -build \
                 -tag neovim:v0.2.0 -build \
-                -tag neovim:v0.4.4 -build
+                -tag neovim:v0.4.4 -build \
+                -tag neovim:v0.5.0 -build
 
 ENV PACKAGES="\
     bash \
     git \
-    python \
-    py-pip \
+    python2 \
+    python3 \
+    py3-pip \
     grep \
     sed \
 "