Ensure build_demo.py run subprocesses with bash shell.
Ensure build_demo.py run subprocesses with bash shell.

It turns out the default shell becomes /bin/sh on Lucid. By specifying the shell for subprocess.check_call we ensure bash is used.

Thanks to yujie.mao@intel.com for pointing this out.

BUG=1659
TEST=Successful build with build_demo.py both on Ubuntu Lucid and Precise.
TBR=leozwang

Review URL: https://webrtc-codereview.appspot.com/1343004

git-svn-id: 917f5d3ca488f358c4d40eaec14422cf392ccec9@3875 4adac7df-926f-26a2-2b94-8c16560cd09d

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