update panic-semihosting to 0.3.0 to use #[panic_implementation]

[tahnok]

Trying to use the f3 crate with rustc 1.28.0-nightly (b68432d56 2018-06-12) fails to compile with

   |
67 | #[lang = "panic_fmt"]
   | ^^^^^^^^^^^^^^^^^^^^^ definition of unknown language item `panic_fmt`

error: aborting due to previous error

Following the recommendation in https://users.rust-lang.org/t/psa-breaking-change-panic-fmt-language-item-removed-in-favor-of-panic-implementation/17875 I bumped panic-semihosting to 0.3.0

[japaric]

Thanks @tahnok.

bors r+

[bors[bot]]

Build failed

• continuous-integration/travis-ci/push

[japaric]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push