Fix .travis.yml indentation
Fix .travis.yml indentation

The standard is two spaces, not one.

digideskio/heapster,titilambert/heapster,wulonghui/heapster,aegixx/heapster,vicki-c/heapster,mdshuai/heapster,shilpamayanna/heapster,shilpamayanna/heapster,vicki-c/heapster,mwielgus/heapster,bruceauyeung/heapster,thucatebay/heapster,titilambert/heapster,taimir/heapster,bluebreezecf/heapster,mdshuai/heapster,Huawei-PaaS/heapster,piosz/heapster,aegixx/heapster,thucatebay/heapster,a-robinson/heapster,luxas/heapster,andyxning/heapster,digideskio/heapster,jackzampolin/heapster,saiwl/heapster,Stackdriver/heapster,burmanm/heapster,whuwxl/heapster,whuwxl/heapster,andyxning/heapster,johanneswuerbach/heapster,mwielgus/heapster,mwringe/heapster,piosz/heapster,taimir/heapster,skytap/heapster,AlmogBaku/heapster,mwringe/heapster,burmanm/heapster,huangyuqi/heapster,adfin/statster,jackzampolin/heapster,adfin/statster,a-robinson/heapster,bruceauyeung/heapster,luxas/heapster,Stackdriver/heapster,saiwl/heapster,afein/heapster,skytap/heapster,AlmogBaku/heapster,wulonghui/heapster,krzyzacy/heapster,johanneswuerbach/heapster,liubog2008/heapster,bluebreezecf/heapster,afein/heapster,liubog2008/heapster,Huawei-PaaS/heapster,krzyzacy/heapster,huangyuqi/heapster