* Prefer python3 over python(2) in checkout-deps
We require Python 3 and don't want to accidentally use Python 2 if both are available.
* Use python3 in travis build
* Install python3-pip package on travis trusty image
* Fix pip detection in checkout-deps.sh
.sh version has this bit:
```
if [ $ismac -eq 0 ]; then
# Add these SDKs for Windows or Linux
sdks+=( orangebox blade episode1 bms )
# Add more SDKs for Windows only
if [ $iswin -eq 1 ]; then
sdks+=( darkm swarm bgt eye contagion )
fi
fi
```
Added these to the SDK list.
