Build failed on Arch Linux

servo/servo#4032 When trying to build servo, build fails at this script. As @SimonSapin mentioned, problem is with Python version. The script assumes version 2.x, whereas on Arch Linux it is 3.x.

```
Build failed, waiting for other jobs to finish...
Failed to run custom build command for `rustuv v0.0.1 (https://github.com/servo/green-rs?ref=servo#9300d7e7)`
Process didn't exit successfully: `make -C build` (status=2)
--- stdout
make: Entering directory '/home/pryx/.cargo/git/checkouts/green-rs-b9cfdf70c4e4296e/servo/librustuv/build'
(cd libuv && python ./gyp_uv.py -f make -Dtarget_arch=x64 \
-D ninja \
-DOS=linux \
-Goutput_dir=/home/pryx/servo/target/native/rustuv-31b2f01c9f5360db/libuv --generator-output /home/pryx/servo/target/native/rustuv-31b2f01c9f5360db/libuv)
Makefile:50: recipe for target '/home/pryx/servo/target/native/rustuv-31b2f01c9f5360db/libuv/Makefile' failed
make: Leaving directory '/home/pryx/.cargo/git/checkouts/green-rs-b9cfdf70c4e4296e/servo/librustuv/build'

--- stderr
File "./gyp_uv.py", line 51
print 'Error running GYP'
^
SyntaxError: Missing parentheses in call to 'print'
make: *** [/home/pryx/servo/target/native/rustuv-31b2f01c9f5360db/libuv/Makefile] Error 1
```