Move dev dependencies to Gemfile.

- This way we have a bit more control over the environment. For example
  ffi is currently not building on travis, but we actually dont need it
  there since guard wont be run.

Gemfile

@@ -8,3 +8,13 @@ platforms :mri_19 do
     gem "debugger"
   end
 end
+
+group :test do
+  gem "mocha", "0.11"
+  gem "rspec", "~> 2.11"
+
+  unless ENV["CI"]
+    gem "guard", "1.2.1"
+    gem "guard-rspec", "~> 0.7"
+  end
+end

mongoid.gemspec

@@ -22,11 +22,6 @@ Gem::Specification.new do |s|
   s.add_dependency("moped", ["~> 1.1.1"])
   s.add_dependency("origin", ["~> 1.0.3"])
 
-  s.add_development_dependency("mocha", ["= 0.11"])
-  s.add_development_dependency("rspec", ["~> 2.10"])
-  s.add_development_dependency("guard", ["= 1.2.1"])
-  s.add_development_dependency("guard-rspec", ["~> 0.7"])
-
   s.files        = Dir.glob("lib/**/*") + %w(CHANGELOG.md LICENSE README.md Rakefile)
   s.require_path = 'lib'
 end