Update unoconv submodule

- To reflect changes in Dag Wieers' master
- Also, change checks for unoconv from read/write to read/execute
Alter checks for unoconv

- We pass unoconv as an argument to python which executes it for us.
- Therefore we don't need to check that if unoconv is directly executable.
Merge pull request #27 from s0600204/office

Update unoconv submodule