Update OLD_RUST_VERSION to 1.29.0

Author: artemmukhin

Diff for: .travis.yml

@@ -10,7 +10,7 @@ jdk: oraclejdk8
 
 env:
   global:
-    - OLD_RUST_VERSION=1.26.0
+    - OLD_RUST_VERSION=1.29.0
     - CURRENT_RUST_VERSION=1.32.0
     - NIGHLY_RUST_VERSION=nightly-2018-12-06
     - RUST_SRC_WITH_SYMLINK=$HOME/.rust-src