Gemfile: pin pg version 0.17

On Rails 4.1.8, pg 0.18.0 cause error.

<pre>
  1) Failure:
WikiContentTest#test_update_with_gzipped_history [test/unit/wiki_content_test.rb:88]:
Expected: "gzip"
  Actual: ""
</pre>

git-svn-id: http://svn.redmine.org/redmine/trunk@13839 e93f8b46-1217-0410-a6f0-8f06a7374b81

Gemfile

@@ -61,7 +61,7 @@ if File.exist?(database_file)
       when 'mysql'
         gem "activerecord-jdbcmysql-adapter", :platforms => :jruby
       when /postgresql/
-        gem "pg", ">= 0.11.0", :platforms => [:mri, :mingw, :x64_mingw]
+        gem "pg", "~> 0.17.1", :platforms => [:mri, :mingw, :x64_mingw]
         gem "activerecord-jdbcpostgresql-adapter", :platforms => :jruby
       when /sqlite3/
         gem "sqlite3", :platforms => [:mri, :mingw, :x64_mingw]