fix some denial of service attacks via hash

Ruby does not GC symbols so there is a possible attack where a large
hash could be passed in and symbolized causing memory bloat. If you are
using `strip: false` the attack is still possible so this is not a
complete solution.
add documentation about a possible security issue
Fix #165; fix RuboCop errors and pin version of RuboCop
oops, wrong version number
Merge branch 'master' into hash-dos
bump version to 1.1.3 and add CHANGELOG notes
fix accidental stringification of non-symbol keys
all filter names are Symbols
bump version to 1.1.4
the transform_keys method is only available in Rails 4.0.2+
bump version to 1.1.5
Merge branch 'master' into v1.2.0