Fix file uploading compatibility via curl for 5.5
Add 5.5 to travis build
Reduce nesting level in AbstractCurl class
Use latest 5.3 branch for travis build
Return empty string instead of `false` on empty response with headers.
Merge pull request #138 from aik099/empty-content-false-fix

Return empty string instead of `false` on empty response with headers
Merge pull request #136 from Koc/file-upload

Fix file uploading compatibility via curl for 5.5