Change Error message for missing subversion

The current error message for missing subversion is "error: please install \`svn\` and try again", leading the user to try to find and install a package named "svn" (eg. `aptitude install svn for # debian / ubuntu). Changing it to "subversion" can help users unaware that svn is a binary installed by the subversion package, get through the error.
Merge pull request #669 from Zorbash/master

Mention 'subversion' in error message when svn is missing
Ignore RUBY_CONFIGURE_OPTS from environment in testing

Avoids failures caused by somebody's environment possibly having
RUBY_CONFIGURE_OPTS exported to have a value.
Fix `script/test` in a non-Travis environment

`script/test` will now only attempt to call `script/mirror` if either
TRAVIS_PULL_REQUEST or TRAVIS_COMMIT_RANGE is set.
Fix error message assertions in JRuby tests

`assert_failure` only accepts an expected message as argument, and only
if it matches the output in full.
Ensure that `assert_output_contains` receives an argument
Copy usage function from rbenv-install to rbenv-uninstall
Abort on wrong number of arguments, resolves #667

rbenv-install accepts 0 or 1 argument
rbenv-uninstall accepts 1 argument
ruby-build accepts 2 arguments
Add tests for wrong number of arguments