Use vim-testbed to run Vader tests on Travis

Dockerfile

@@ -0,0 +1,13 @@
+FROM tweekmonster/vim-testbed:latest
+
+RUN install_vim -tag v8.0.0000 -build \
+                -tag v8.0.0027 -build
+
+ENV PACKAGES="\
+    git \
+"
+RUN apk --update add $PACKAGES && \
+    rm -rf /var/cache/apk/* /tmp/* /var/tmp/*
+
+RUN git clone https://github.com/junegunn/vader.vim vader && \
+    cd vader && git checkout c6243dd81c98350df4dec608fa972df98fa2a3af