Merge pull request #606 from sass/test/travis-ci

[WIP] Fixes and improves travis ci build
Removes redundant builds in travis-ci config

Further speeds up day by day travis-ci builds
This will still catch all coding errors!
Support function and mixin calls with both variable and keyword args
Reevaluate var and keyword arg flags after eval a \$variable argument

```
foo($bar...);
```

Will flag `$bar` as a variable arg when in fact it may be a keyword
arg. We can only be sure once `$bar` has been evaluated. If `$bar`
is evaluated to a map then it's a keyword argument and the
Argument object needs to be updated accordingly.
Merge pull request #611 from xzyfer/feat/map-varargs

Add support for keyword arguments
Convert tabs to white spaces

...to make PR diffs cleaners/
Add basic support for feature queries
Merge pull request #612 from xzyfer/feat/at-supports

Add basic support for feature queries
Fix typo
Hoist feature query's selector
Merge pull request #618 from xzyfer/feat/hoist-feature-queries

Hoist selectors in feature queries
Adds missing options on sass_interfaces
[BUGFIX] Add fix for source-maps (Fix-Up PR #591)

- Remove line-breaks in Base64 encoded string
- Actually embed the source-map json in data-url
- Include original source in sources array
Merge pull request #619 from sass/bugfix/source-map-sources

[BUGFIX] Add fix for source-maps (Fix-Up PR #591)
Correctly handle nested feature queries
Merge pull request #626 from xzyfer/fix/nested-feature-queries

Correctly handle nested feature queries
Fix the hanging } in some at-rules nested output
Merge pull request #627 from xzyfer/fix/at-block-nested-output

Fix the hanging } in some at-rules nested output
Handles leading zeros (numbers) as seen in input
Merge pull request #624 from mgreter/issue/550

Output leading zeros on numbers as seen in input (Fixes #550)