Add / to the list of characters to escape

This is a fix for migration from elasticsearch 0.20.6 to 0.90+.
Elasticsearch 0.90+ uses Lucene 4.  One change in Lucene 4 from Lucene 3
is that a / character in a query denotes the start of a regular
expression.  We don't want to use regular expressions in our queries, so
add / to the list of characters to escape.  This change fixes the
test_should_not_match_on_slug integration test.

For more information:
https://github.com/elasticsearch/elasticsearch/issues/2980
Merge pull request #221 from alphagov/regex_fix_for_0_90

Add / to the list of characters to escape