Update bitflags from 0.9 to 1.2

[ackintosh]

v0.9 causes build warnings due to the usage of deprecated item try!:

$ cargo build

   Compiling parity-whisper v0.1.0 (/Users/akihito1/src/github.com/ackintosh/whisper)
warning: use of deprecated item 'try': use the `?` operator instead
  --> src/rpc/payload.rs:45:1
   |
45 | / bitflags! {
46 | |     struct Flags: u8 {
47 | |         const FLAG_PAD_LEN_HIGH = 0b10000000;
48 | |         const FLAG_PAD_LEN_LOW  = 0b01000000;
49 | |         const FLAG_SIGNED       = 0b00100000;
50 | |     }
51 | | }
   | |_^
   |
   = note: `#[warn(deprecated)]` on by default
   = note: this warning originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

CHANGELOG: https://github.com/bitflags/bitflags/blob/master/CHANGELOG.md