Fix PATH to use the fully generic binstubs

[yosifkit]

bundler binstubs are not necessarily generic; they were until bundler 1.16 and should be fixed back, but will likely be non-generic again by default in 2.0

See rubygems/bundler#6469 (comment) for more context.

Fixes #188. Fixes #191. Closes #193.

I have successfully built the following Dockerfiles using this change that fail in the current ruby images:

FROM ruby:2.4-test
RUN set -ex;\
    git clone https://github.com/jwt/ruby-jwt.git;\
    cd ruby-jwt;\
    bundle install
RUN set -ex;\
    git clone https://github.com/janlelis/clipboard.git;\
    cd clipboard;\
    bundle install;\
    ls -la /clipboard/Gemfile.lock


FROM ruby:2.4-test
RUN set -ex;\
    git clone https://github.com/jwt/ruby-jwt.git;\
    cd ruby-jwt;\
    bundle install;\
    cd ..;\
    hash -r; \
    git clone https://github.com/janlelis/clipboard.git;\
    cd clipboard;\
    bundle install;\
    ls -la /clipboard/Gemfile.lock

[tianon]

Can we include a link in the Dockerfile itself to rubygems/bundler#6469 (comment)? 😇

[tianon]

It was green before the comment addition, merging. 👍

[tianon]

This change removes $GEM_HOME/bin from PATH, so gem install xyz no longer puts binaries in PATH -- perhaps we need to couple this with no longer setting BUNDLE_BIN, and add $GEM_HOME/bin to PATH explicitly too.

[tianon]

#209 👍