Update Dockerfile to test NeoVim 0.2 and 0.3, with a newer Vint version

Dockerfile

@@ -1,7 +1,8 @@
 FROM tweekmonster/vim-testbed:latest
 
 RUN install_vim -tag v8.0.0027 -build \
-                -tag neovim:v0.1.7 -build
+                -tag neovim:v0.2.0 -build \
+                -tag neovim:v0.3.0 -build
 
 ENV PACKAGES="\
     bash \
@@ -12,7 +13,7 @@ ENV PACKAGES="\
 RUN apk --update add $PACKAGES && \
     rm -rf /var/cache/apk/* /tmp/* /var/tmp/*
 
-RUN pip install vim-vint==0.3.9
+RUN pip install vim-vint==0.3.15
 
 RUN git clone https://github.com/junegunn/vader.vim vader && \
     cd vader && git checkout c6243dd81c98350df4dec608fa972df98fa2a3af