add travis for CI testing
Merge remote-tracking branch 'upstream/master'
transforms: add 'unique' to filter unique rows (by single header)
transforms: Unique - support for multiple headers
transforms: Unique - test multiple rows returned
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' into unique

Conflicts:
	transforms/transforms.go
	transforms/transforms_test.go
transforms: unique - use Select to read rows
transforms: unique - use Select to read rows (fix syntax derp in test)
travis: remove, will use drone instead
Merge pull request #3 from nathanleiby/unique

Unique
v1.1.2: add Unique transform
sinks: csv: write nil as empty string
sinks: csv: document assumption