Switch to monotonic clock

[stephanie-wang]

This PR switches from std::chrono::system_clock to std::chrono::steady_clock, so that current_time will always be monotonically increasing. This should fix these occasional Travis errors:

[FATAL] (/home/travis/build/ray-project/ray/src/plasma/plasma_manager.cc:1476: errno: Operation now in progress) Check failure: current_time >= state->previous_heartbeat_time 

/home/travis/.local/lib/python2.7/site-packages/ray-0.2.1-py2.7-linux-x86_64.egg/ray/plasma/../core/src/plasma/plasma_manager[0x4041ac]
/home/travis/.local/lib/python2.7/site-packages/ray-0.2.1-py2.7-linux-x86_64.egg/ray/plasma/../core/src/plasma/plasma_manager[0x421084]
/home/travis/.local/lib/python2.7/site-packages/ray-0.2.1-py2.7-linux-x86_64.egg/ray/plasma/../core/src/plasma/plasma_manager[0x42131b]
/home/travis/.local/lib/python2.7/site-packages/ray-0.2.1-py2.7-linux-x86_64.egg/ray/plasma/../core/src/plasma/plasma_manager[0x408a56]
/home/travis/.local/lib/python2.7/site-packages/ray-0.2.1-py2.7-linux-x86_64.egg/ray/plasma/../core/src/plasma/plasma_manager[0x403a02]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf5)[0x7f1712ffdf45]
/home/travis/.local/lib/python2.7/site-packages/ray-0.2.1-py2.7-linux-x86_64.egg/ray/plasma/../core/src/plasma/plasma_manager[0x403fe7]
[FATAL] (/home/travis/build/ray-project/ray/src/local_scheduler/local_scheduler.cc:1218: errno: Operation now in progress) Check failure: current_time >= state->previous_heartbeat_time 

[AmplabJenkins]

Merged build finished. Test PASSed.

[AmplabJenkins]

Test PASSed.
Refer to this link for build results (access rights to CI server needed):
https://amplab.cs.berkeley.edu/jenkins//job/Ray-PRB/2078/
Test PASSed.