travis: Set min-rust-version to 1.45

Some dev-dependencies have increaed their min-rust-version to 1.38.0 or
later, which has been breaking CI.

Also, `UNICODE_VERSION` is stabilized in 1.45.0, and we're going to use
that in non-nightly builds now.

.travis.yml

@@ -7,7 +7,7 @@ rust:
   - nightly
   - beta
   - stable
-  - 1.31.0  # = MIN_RUST_VERSION
+  - 1.45.0  # = MIN_RUST_VERSION
 
 branches:
   except:

README.md

@@ -4,7 +4,7 @@
 
 [![Travis](https://img.shields.io/travis/open-i18n/rust-unic/master.svg?label=Linux%20build)](https://travis-ci.org/open-i18n/rust-unic/)
 [![AppVeyor](https://img.shields.io/appveyor/ci/open-i18n/rust-unic/master.svg?label=Windows%20build)](https://ci.appveyor.com/project/open-i18n/rust-unic)
-[![Rust-1.31.0+](https://img.shields.io/badge/rustc-1.31+-red.svg#MIN_RUST_VERSION)](https://www.rust-lang.org/)
+[![Rust-1.45+](https://img.shields.io/badge/rustc-1.45+-red.svg#MIN_RUST_VERSION)](https://www.rust-lang.org/)
 [![Unicode-10.0.0](https://img.shields.io/badge/unicode-10.0.0-red.svg)](https://www.unicode.org/versions/Unicode10.0.0/)
 [![Release](https://img.shields.io/github/release/open-i18n/rust-unic.svg)](https://github.com/open-i18n/rust-unic/)
 [![Crates.io](https://img.shields.io/crates/v/unic.svg)](https://crates.io/crates/unic/)