Update required Rust version to 1.8.0
Update required Rust version to 1.8.0

Fixes #254
mattico/rusoto,DualSpark/rusoto,mattico/rusoto,SecurityInsanity/rusoto,indiv0/rusoto,rusoto/rusoto,SecurityInsanity/rusoto,DualSpark/rust-aws,noggleboggle/rusoto,DualSpark/rusoto,mattico/rusoto,cmsd2/rusoto,DualSpark/rust-aws,mthjones/rusoto,mthjones/rusoto,DualSpark/rust-aws,DualSpark/rusoto,rusoto/rusoto,ford-prefect/rusoto,ford-prefect/rusoto,rusoto/rusoto