Raise minimum rustc version to 1.28 (Fixes CI)

Known minimum version to build [regex](https://crates.io/crates/regex).
Old enough as it is, we're not c++ here!

parser/.travis.yml

@@ -5,9 +5,7 @@ matrix:
     - rust: stable
     - rust: beta
     - rust: nightly
-    - rust: 1.17.0
-      script: cargo build
-    - rust: 1.24.1
+    - rust: 1.28.0
     - rust: nightly
       env: CLIPPY
       script: |

parser/README.md

@@ -103,6 +103,10 @@ so it may not be a huge problem for most users.
 * Tag directive
 * Alias while desearilization
 
+## Minimum Rust version policy
+
+This crate's minimum supported `rustc` version is 1.28, as this is the currently known minimum version for [`regex`](https://crates.io/crates/regex#minimum-rust-version-policy) as well.
+
 ## License
 
 Licensed under either of

parser/appveyor.yml

@@ -1,6 +1,6 @@
 install:
-  - ps: Start-FileDownload 'https://static.rust-lang.org/dist/rust-1.24.1-i686-pc-windows-gnu.exe'
-  - rust-1.24.1-i686-pc-windows-gnu.exe /VERYSILENT /NORESTART /DIR="C:\Program Files (x86)\Rust"
+  - ps: Start-FileDownload 'https://static.rust-lang.org/dist/rust-1.28.0-i686-pc-windows-gnu.exe'
+  - rust-1.28.0-i686-pc-windows-gnu.exe /VERYSILENT /NORESTART /DIR="C:\Program Files (x86)\Rust"
   - SET PATH=%PATH%;C:\Program Files (x86)\Rust\bin
   - SET PATH=%PATH%;C:\MinGW\bin
   - rustc -V