Bump proc macros 2 module to latest version #13

Bug report :

There was a recent change in proc macros 1.2 API that made some breaking changes. It broke proc macros 2 with version 0.3.8 on some projects. So I tried running this project against latest nightly and it works which should have caused an error during compilation of proc macros 2 module.

Upstream rustc issue reference : rust-lang/rust#50473 (comment)
Upstream proc macros 2 PR (merged) : dtolnay/proc-macro2#90
Successful Nightly build with 0.3.8 compilation : https://travis-ci.org/tirkarthi/num-derive/jobs/382026352

Failure of proc macros 0.3.8 compilation in Rocket : https://travis-ci.org/SergioBenitez/Rocket/jobs/381529609#L527
proc macros 2 v0.4.3 released with the changes. Ref : https://github.com/alexcrichton/proc-macro2/tree/0.4.3

Feel free to close this if it's irrelevant or I am missing something.

Thanks