Merge pull request #765 from eli-schwartz/portable-command-v

Use portable "command -v" to detect installed programs

lib/travis/tools/system.rb

@@ -76,7 +76,7 @@ def description(*args)
       def has?(command)
         return false unless unix?
         @has ||= {}
-        @has.fetch(command) { @has[command] = system "which #{command} 2>/dev/null >/dev/null" }
+        @has.fetch(command) { @has[command] = system "command -v #{command} 2>/dev/null >/dev/null" }
       end
 
       def running?(app)