Bump minimum supported rust version to 1.13

kaksmet · kaksmet · commit 4b1585ad85d6 · 2017-09-03T00:16:38.000+02:00
Some of our dependencies now relies on `?` which was stabilized in 1.13.
diff --git a/.travis.yml b/.travis.yml
@@ -1,6 +1,6 @@
 language: rust
 rust:
-  - 1.12.0
+  - 1.13.0
   - stable
   - beta
   - nightly

-Original file line number
+Diff line change
@@ @@ -1,6 +1,6 @@ @@
 language: rust
 rust:
 -  - 1.12.0
 +  - 1.13.0
   - stable
   - beta
   - nightly