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.
changed wording in comment as suggested by @Zorbash
fail ruby-build displaying usage if number of arguments is not equal to 2
copy usage function from rbenv-install to rbenv-uninstall
use usage to fail on wrong number of arguments in rbenv-install and rbenv-uninstall
use ARGUMENTS array in rbenv-install when testing number of arguments
add tests for wrong number of arguments