Handle missing postgresql-contrib gracefully

A user encountered this error on Centos 6.7 while upgrading to 4.2.0 from the previous version:
http://pastebin.com/UEB5YSxQ

looking at the postgres logs showed

14:54 <jdehnert> From postgres.. < 2016-08-10 14:03:04.523 PDT >ERROR: could not access file "$libdir/pg_trgm": No such file or directory
14:54 <jdehnert> < 2016-08-10 14:03:04.523 PDT >STATEMENT: CREATE INDEX fact_values_string_trgm ON fact_values USING gin (value_string gin_tr
14:54 <jdehnert> gm_ops)

Installing postgresql-contrib resolved it, but we need to make sure we're handling the case where that package is missing and also figure out why (sutils/pg-extension? "pg_trgm") returned truthy.