deps: require regex-syntax 0.6.14
deps: require regex-syntax 0.6.14

This brings in the bug fix for #640 for flag scoping.

rust-lang/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex