Add support for external ruby deps, and fix external deps system.

Kernel#system special-cases the first argument, so you have to
make the first argument the entire command to be invoked, and
subsequent arguments the actual arguments to that command. In
order to use the user's interpreter, the first argument must be
"/usr/bin/env <name>".

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Library/Homebrew/formula_installer.rb

@@ -27,7 +27,6 @@ Unsatisfied dependency, #{dep}
 Homebrew does not provide formula for Python dependencies, pip does:
 
    #{brew_pip} pip install #{dep}
-
     EOS
   end
   def plerr dep; <<-EOS
@@ -35,7 +34,13 @@ Unsatisfied dependency, #{dep}
 Homebrew does not provide formula for Perl dependencies, cpan does:
 
     cpan -i #{dep}
+    EOS
+  end
+  def rberr dep; <<-EOS
+Unsatisfied dependency "#{dep}"
+Homebrew does not provide formulae for Ruby dependencies, rubygems does:
 
+    gem install #{dep}
     EOS
   end
 
@@ -43,24 +48,32 @@ Homebrew does not provide formula for Perl dependencies, cpan does:
     return unless f.external_deps
 
     f.external_deps[:python].each do |dep|
-      raise pyerr(dep) unless quiet_system "/usr/bin/python", "-c", "import #{dep}"
+      raise pyerr(dep) unless quiet_system "/usr/bin/env", "python", "-c", "import #{dep}"
     end
     f.external_deps[:perl].each do |dep|
-      raise plerr(dep) unless quiet_system "/usr/bin/perl", "-e", "use #{dep}"
+      raise plerr(dep) unless quiet_system "/usr/bin/env", "perl", "-e", "use '#{dep}'"
+    end
+    f.external_deps[:ruby].each do |dep|
+      raise rberr(dep) unless quiet_system "/usr/bin/env", "ruby", "-rubygems", "-e", "require '#{dep}'"
     end
   end
 
-  def install f
+  def check_formula_deps f
     expand_deps(f).each do |dep|
       begin
-        check_external_deps f
         install_private dep unless dep.installed?
       rescue
         #TODO continue if this is an optional dep
         raise
       end
-    end if @install_deps
+    end
+  end
 
+  def install f
+    if @install_deps
+      check_external_deps f
+      check_formula_deps f
+    end
     install_private f
   end