Fix Python CI: More swap space

[gchenfc]

Recent CI's have been failing due to Python compile needing large amounts of RAM. Specifically, gcc in DEBUG mode appears to be pushing the CI over the edge. e.g.

[ 93%] Built target gtsam_unstable_py
[ 93%] Building CXX object python/CMakeFiles/gtsam_py.dir/slam.cpp.o
g++-5: internal compiler error: Killed (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See file:///usr/share/doc/gcc-5/README.Bugs for instructions.
make[2]: *** [python/CMakeFiles/gtsam_py.dir/nonlinear.cpp.o] Error 4
python/CMakeFiles/gtsam_py.dir/build.make:257: recipe for target 'python/CMakeFiles/gtsam_py.dir/nonlinear.cpp.o' failed
make[2]: *** Waiting for unfinished jobs....
CMakeFiles/Makefile2:6887: recipe for target 'python/CMakeFiles/gtsam_py.dir/all' failed
make[1]: *** [python/CMakeFiles/gtsam_py.dir/all] Error 2
make: *** [all] Error 2
Makefile:165: recipe for target 'all' failed
Error: Process completed with exit code 2.

Varun/Fan/I discussed this issue.

This PR increases the (Linux) swap space from (default) 4GB to 6GB. Alternatively, we could also reduce number of threads from -j2 to -j1 but that will ~double build times.

Also for reference, currently hard drive usage is very low, only 4.1G / 14G. Here's output of df after python DEBUG build on python CI:

runner@fv-az199-378:~/work/gtsam/gtsam$ df -h
Filesystem      Size  Used Avail Use% Mounted on
udev            3.4G     0  3.4G   0% /dev
tmpfs           696M  740K  695M   1% /run
/dev/sdb1        84G   59G   25G  71% /
tmpfs           3.4G     0  3.4G   0% /dev/shm
tmpfs           5.0M     0  5.0M   0% /run/lock
tmpfs           3.4G     0  3.4G   0% /sys/fs/cgroup
/dev/sdb15      105M  4.4M  100M   5% /boot/efi
/dev/sda1        14G  4.1G  9.0G  31% /mnt
tmpfs           696M     0  696M   0% /run/user/1001

[varunagrawal]

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