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ARROW-1253: [C++/Python] Speed up C++ / Python builds by using conda-forge toolchain for thirdparty libraries

@pcmoritz I removed the travis_script_plasma.sh and combined the common bits with travis_script_python.sh; it seemed like there was some redundant work and would be easier to test everything in a single build. Let me know if I messed something up

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #884 from wesm/ARROW-1253 and squashes the following commits:

30a13ee3 [Wes McKinney] Formatting
28bc6be8 [Wes McKinney] Run unit tests from installed library
fcf7940d [Wes McKinney] Prevent existing LD_LIBRARY_PATH from being blown away
c67bb68b [Wes McKinney] Speed up C++ / Python builds by using conda-forge toolchain to avoid rebuilding EPs. Use ninja for builds when using toolchain