Replace tab indentation from travis.yml file

Error description:
```
ERROR: An error occured while trying to parse your .travis.yml file.
Please make sure that the file is valid YAML.
The error was "found character 	 '\t(TAB)' that cannot start any token.
(Do not use \t(TAB) for indentation) while scanning for the next token
at line 5 column 1".
```