Temporary fix for rb 1.8.7 on Travis CI

Ruby 1.8.7 is broken otherwise, but doing this fixes it. No harm in running it for other rubies either.

See https://github.com/bundler/bundler/issues/2784 and https://github.com/travis-ci/travis-ci/issues/1793 for more context on the bug.

We should revert this once Travis has fixed it and rolled the fix out.
Merge pull request #91 from guilleiguaran/travis-1-8-7

Temporary fix for rb 1.8.7 on Travis CI
Adds robustness to the zremrangebyrank method to not error when the set is missing (since the set may have been removed by remove_key_for_empty_collection because it was empty)
Adds support for exclusive ranges in zrangebyscore to match current Redis capability
allow srandmembers to take an optional parameter for the number of elements

* with negative numbers, redis supports repeated samplings
* behavior with empty sets is different: empty array instead of nil
change implementation for old rubies
add .time to FakeRedis
add timecop for testing .time
Merge pull request #98 from xgess/add-support-for-multiple-srandmembers

allow srandmembers to take an optional parameter for the number of eleme...
Support the SCAN command

http://redis.io/commands/scan
replace timecop with stubbing time for 1.8.7 support
make fakeredis conform to same setbit api as redis
Merge pull request #101 from xgess/setbit-return-integers

make fakeredis conform to same setbit api as redis
Merge pull request #100 from jasonroelofs/support-redis-scan

Support the SCAN command
Merge pull request #96 from timherby/support-exclusive-min-max

Adds support for exclusive ranges in zrangebyscore
Merge pull request #99 from xgess/add-time

add .time to FakeRedis
Only run stubbed time specs against fakeredis

Obviously when we run them against the real redis server, it returns the real time, not our stubbed version.
Use travis-recommended rbx Gemfile config
Add ruby 2.1.1 to travis config
Merge branch 'update-travis-config'