gnu: Use INVOKE & return #t from Ruby package phases.

gnu: Use INVOKE & return #t from Ruby package phases.

* gnu/packages/maths.scm (ruby-asciimath)[arguments]: Return #t from
all phases.
* gnu/packages/rails.scm (ruby-debug-inspector)[arguments]: Substitute
INVOKE for SYSTEM*.
* gnu/packages/ruby.scm (ruby-permutation, ruby-blankslate)
(ruby-minitest-pretty-diff, ruby-guard, ruby-rb-inotify, ruby-tins)
(ruby-gem-hadar, ruby-term-ansicolor, ruby-json-pure)
(ruby-activesupport, ruby-redcloth, ruby-netrc, ruby-unf-ext, ruby-yard)
(ruby-turn, ruby-sqlite3, ruby-shoulda-context, ruby-shoulda-matchers)
(ruby-shoulda, ruby-httpclient, ruby-ansi, ruby-bio-commandeer)
(ruby-rubytest, ruby-brass, ruby-qed, ruby-ae, ruby-lemon, ruby-hashery)
(ruby-rc4, ruby-concurrent)[arguments]: Likewise.