Removing it triggers the following warnings when running `bundle` under
jruby from the root of the `psych` repo prints the following warnings:
```
/path/to/jruby-9.2.9.0/lib/ruby/gems/shared/gems/psych-3.1.0-java/lib/psych/versions.rb:7: warning: already initialized constant VERSION
/path/to//jruby-9.2.9.0/lib/ruby/gems/shared/gems/psych-3.1.0-java/lib/psych/versions.rb:10: warning: already initialized constant DEFAULT_SNAKEYAML_VERSION
```
This is because bundler loads the versions file relatively from the
local gemspec, and then internally loads the psych gem, causing the
redefinition warnings.
Instead, we modify the $LOAD_PATH so that when working locally on the
`psych` repo, the local version of `psych` gets used.
https://github.com/ruby/psych/commit/a3fc8191a7
cd3d035e8a