Update sequel dependency in bundle to fork

this fork has patched the tests to ignore tests that don't work
with fdbsql. Maybe it will get merged upstream and we won't have
to point to a fork anymore
Added test skipped in Sequel proper

the sequel test caused an error, but we can still do some of the
test, so I added it here
Updated RUNNING_UNIT_TESTS.md
Added comment
Updated Sequel version in README
Changed dependencies to 4.13.0
Added missing method
Merge pull request #7 from ScottDugas/ignore-failing-tests

Ignore failing tests
Remove open ended rake requirement
Allow versions 4.12 and 4.13
Added note about past_version potential
Updated README that we support 4.12 & 4.13
Use tag for fdbsql-4.13.0
Merge pull request #8 from ScottDugas/4.13.0

Support for sequel 4.13.0
Moved connection.rb into fdbsql.rb
Moved dataset.rb into fdbsql.rb
Moved features.rb into fdbsql.rb
Moved prepared_statements.rb into fdbsql.rb
Removed old include calls
Moved date_arithmetic.rb into fdbsql.rb