Correct use of promises.

[OJezu]

fixes #118

As mentioned in #131, current tip (before this merge) causes following error:

Error in plugin 'gulp-less'
Message:
    'font-awesome/less/variables.less' wasn't found. Tried - /home/ch/src/Browser/style/font-awesome/less/variables.less,font-awesome/less/variables.less in file /home/ch/src/Browser/style/config.less line no. 1
Details:
    type: File
    filename: /home/ch/src/Browser/style/config.less
    index: 0
    line: 1
    callLine: NaN
    callExtract: undefined
    column: 0
    extract: ,@import "font-awesome/less/variables";,
    lineNumber: 1
    fileName: /home/ch/src/Browser/style/config.less

Please, be wary.

[yocontra]

Fixed in 2.0.3