Set Makefile default goal to `tests` instead of `clean`
Set Makefile default goal to `tests` instead of `clean`

By virtue of being the first goal in the Makefile, `clean` is currently
the default. Personally I feel this is slightly aggressive (having
accidentally wiped out scratch files twice now due to bare `make`s) so
this sets `.DEFAULT_GOAL` to `tests`, which is non-destructive and
(based on no data other than my own usage) probably more desired.

icio/github3.py,itsmemattchung/github3.py,sigmavirus24/github3.py,jim-minter/github3.py,agamdua/github3.py,wbrefvem/github3.py,balloob/github3.py,degustaf/github3.py,h4ck3rm1k3/github3.py,krxsky/github3.py,christophelec/github3.py,ueg1990/github3.py