Use #!/bin/bash for Travis build script which requires bash

It uses arithmetic expansion and, now, arrays, neither of which exists
in POSIX sh.

.travis.yml

@@ -108,4 +108,4 @@ notifications:
 
 before_install: ./build/tools/before_install.sh
 
-script: bash build/tools/travis-ci.sh
+script: ./build/tools/travis-ci.sh

build/tools/travis-ci.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # This script is used by Travis CI to configure, build and test wxWidgets