Optimized Metric::CiMixin#has_perf_data? method

https://bugzilla.redhat.com/show_bug.cgi?id=1086015

has_perf_data? method calls ActiveRecord exists? instead of
calling first_capture (which calls first_and_last_capture).
Merge pull request #130 from chessbyte/bz1086015_optimize_has_perf_data_method

Optimized Metric::CiMixin#has_perf_data? method