Use Mustermann from master until a release is made

Since master has a patch which fixes the missing `:+` implementation for
Regular and Sinatra-based patterns.

We can also re-enable the namespace specs for accepting regular expressions for
path concatination.

/cc #200 and #207

Gemfile

@@ -1,6 +1,8 @@
 source "https://rubygems.org" unless ENV['QUICK']
 gemspec
 
+gem 'mustermann', github: 'sinatra/mustermann'
+
 group :development, :test do
 
   platform :ruby_18 do

lib/sinatra/namespace.rb

@@ -268,9 +268,7 @@ def compile(pattern, conditions, default_pattern = nil)
       def prefixed_path(a, b)
         return a || b || /.*/ unless a and b
 
-        Mustermann.new(a.to_s + b.to_s)
-      rescue
-        Mustermann.new(/#{a}#{b}/)
+        Mustermann.new(a) + Mustermann.new(b)
       end
 
       def prefixed(method, pattern = nil, conditions = {}, &block)

spec/namespace_spec.rb

@@ -80,8 +80,6 @@ def namespace(*args, &block)
         end
 
         it 'accepts the path as regular expression' do
-          skip "Sinatra::Namespace accepts regular expressions for pattern"
-
           namespace('/:foo') { send(verb, %r{/bar}) { params[:foo] }}
           send(verb, '/foo/bar').should be_ok
           body.should == 'foo' unless verb == :head
@@ -99,8 +97,6 @@ def namespace(*args, &block)
         end
 
         it 'accepts the path as a named parameter' do
-          skip "Sinatra::Namespace accepts regular expressions for pattern"
-
           namespace(%r{/foo}) { send(verb, '/:bar') { params[:bar] }}
           send(verb, '/foo/bar').should be_ok
           body.should == 'bar' unless verb == :head