Bump minimum rust version up to 1.28.0.

oherrala · oherrala · commit 6ccfb4ce05e1 · 2019-09-10T00:58:14.000+03:00
This is the oldest version regex 1.3 crate supports.
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,7 @@
 language: rust
 sudo: false
 rust:
-  - 1.24.1
+  - 1.28.0
   - stable
   - beta
   - nightly

-Original file line number
+Diff line change
@@ @@ -1,7 +1,7 @@ @@
 language: rust
 sudo: false
 rust:
 -  - 1.24.1
 +  - 1.28.0
   - stable
   - beta
   - nightly