text: Title: sh: gmm-boost-silence: command not found when running gmm-aligner.py
  Body: pykaldi was installed and `python setup.py test` shows that it either skipped or passed all the tests. When I ran the example script examples/alignment/gmm-aligner.py I get the error below.
  
  ```
  sh: gmm-boost-silence: command not found
  ERROR ([5.5.195~1-9daa6]:ExpectToken():io-funcs.cc:203) Failed to read token [started at file position -1], expected <TransitionModel>
  ERROR ([5.5.195~1-9daa6]:ExpectToken():io-funcs.cc:203) Failed to read token [started at file position -1], expected <TransitionModel>
  WARNING ([5.5.195~1-9daa6]:Close():kaldi-io.cc:515) Pipe gmm-boost-silence --boost=1.0 1 final.mdl - | had nonzero return status 32512
  Traceback (most recent call last):
    File "/home/jpadmin/Desktop/pycharm-community-2018.1.4/helpers/pydev/pydevd.py", line 1664, in <module>
      main()
    File "/home/jpadmin/Desktop/pycharm-community-2018.1.4/helpers/pydev/pydevd.py", line 1658, in main
      globals = debugger.run(setup['file'], None, None, is_module)
    File "/home/jpadmin/Desktop/pycharm-community-2018.1.4/helpers/pydev/pydevd.py", line 1068, in run
      pydev_imports.execfile(file, globals, locals)  # execute the script
    File "/media/jpadmin/c92d86f3-ed84-4b98-b4ca-3d7777aca034/kaldi/pykaldi/examples/alignment/gmm-aligner.py", line 13, in <module>
      self_loop_scale=0.1)
    File "/media/jpadmin/c92d86f3-ed84-4b98-b4ca-3d7777aca034/kaldi/pykaldi/kaldi/alignment.py", line 403, in from_files
      transition_model, acoustic_model = cls.read_model(model_rxfilename)
    File "/media/jpadmin/c92d86f3-ed84-4b98-b4ca-3d7777aca034/kaldi/pykaldi/kaldi/alignment.py", line 369, in read_model
      ki.binary)
  RuntimeError: C++ exception: 
  ```
  The strange thing is that I am able to run gmm-boost-slience from the terminal and `which` shows that it's pointing to the kaldi directory inside tools
  
  >/media/jpadmin/c92d86f3-ed84-4b98-b4ca-3d7777aca034/kaldi/pykaldi/tools/kaldi/src/gmmbin/gmm-boost-silence
  
  I have already added KALDI_ROOT to path in bashrc 
  
  ```
  export KALDI_ROOT='/media/jpadmin/c92d86f3-ed84-4b98-b4ca-3d7777aca034/kaldi/pykaldi/tools/kaldi'
  export PATH=$KALDI_ROOT/src/bin:$KALDI_ROOT/tools/openfst/bin:$KALDI_ROOT/src/fstbin:$KALDI_ROOT/src/gmmbin:$KALDI_ROOT/src/featbin:$KALDI_ROOT/src/lmbin:$KALDI_ROOT/src/sgmm2bin:$KALDI_ROOT/src/fgmmbin:$KALDI_ROOT/src/latbin:$PATH
  ```
  I am not sure why gmm-boost-silence is not being found within GmmAligner.from_files()
  
  
  