Travis: new JRuby, develop on 2.4.0 (#476)

* Travis: new JRuby, develop on 2.4.0
* CircleCI: ensure new rubygems
* Rubocop: RegexpMatch too edgy; long blocks OK in specs
* Travis: JRuby head fails bundle install
* Travis: Allow failures for jruby-head

.rubocop.yml

@@ -10,6 +10,9 @@ AllCops:
 Metrics/LineLength:
   Enabled: false
 
+Performance/RegexpMatch:
+  Enabled: false
+
 #http://viget.com/extend/just-use-double-quoted-ruby-strings
 Style/StringLiterals:
   EnforcedStyle: double_quotes
@@ -66,4 +69,8 @@ Style/NumericPredicate:
   Enabled: false
 
 Style/SafeNavigation:
-  Enabled: false
+  Enabled: false
+
+Metrics/BlockLength:
+  Exclude:
+    - 'spec/**/*'