Fix Boolean flag when next value contains the strings 'true' or 'false'

[elas7]

When calling test.js -b aaafalseaaa, and b is a boolean, the value of 'b' is set to false and 'aaafalseaaa' doesn't appear in '_'.

I fixed this by updating the regex to look like the one in the --foo true case, where it works fine.

[coveralls]

Coverage remained the same at 100.0% when pulling ca533db on elas7:fix-bool into 48b1e6a on yargs:master.

[bcoe]

@elas7 mind rebasing 👍

[coveralls]

Coverage remained the same at 100.0% when pulling 69b18db on elas7:fix-bool into 90b871f on yargs:master.

[bcoe]

great fix 👍