fix AttributeError: 'MakePdfCommand' object has no attribute 'envi'

Looks like self.envi has to be defined in all cases... so just simply initialized it with os.environ.copy() at the beginning 

```
Traceback (most recent call last):
File "X/threading.py", line 639, in _bootstrap_inner
File "/home/.../.config/sublime-text-3/Packages/LaTeXTools/makePDF.py", line 53, in run
env = self.caller.envi.copy()
AttributeError: 'MakePdfCommand' object has no attribute 'envi'
```