Fix for bad command line argument to latex

When running "iptest IPython.lib" on master dafe88e I get the errors below. I'm using Windows 7 python2.7.

The option to latex is bad it should be -halt-on-error not -halt-on-errror.
However when changing the option the test suite locks up on the third
"Test that latex_to_png_dvipng just runs without error." test.

I suspect it is related to the warning in the documentation of check_call

"Like Popen.wait(), this will deadlock when using stdout=PIPE and/or stderr=PIPE and the child process generates   enough output to a pipe such that it blocks waiting for the OS pipe buffer to accept more data."


Errors:

	C:\python27\external> iptest IPython.lib
	..............................EEEE..............
	======================================================================
	ERROR: Test that latex_to_png_dvipng just runs without error.
	----------------------------------------------------------------------
	Traceback (most recent call last):
	  File "c:\python27\lib\site-packages\nose\case.py", line 197, in runTest
	    self.test(*self.arg)
	  File "c:\python27\lib\site-packages\ipython-0.14.dev-py2.7.egg\IPython\lib\latextools.py", line 137, in latex_to_png_dvipng
	    stdout=subprocess.PIPE, stderr=subprocess.PIPE)
	  File "c:\python27\lib\subprocess.py", line 504, in check_call
	    raise CalledProcessError(retcode, cmd)
	CalledProcessError: Command '['latex', '-halt-on-errror', 'c:\\users\\jorgenst\\appdata\\local\\temp\\tmp0u_vjx\\tmp.tex']' returned non-zero exit status 1

	======================================================================
	ERROR: Test that latex_to_png_dvipng just runs without error.
	----------------------------------------------------------------------
	Traceback (most recent call last):
	  File "c:\python27\lib\site-packages\nose\case.py", line 197, in runTest
	    self.test(*self.arg)
	  File "c:\python27\lib\site-packages\ipython-0.14.dev-py2.7.egg\IPython\lib\latextools.py", line 137, in latex_to_png_dvipng
	    stdout=subprocess.PIPE, stderr=subprocess.PIPE)
	  File "c:\python27\lib\subprocess.py", line 504, in check_call
	    raise CalledProcessError(retcode, cmd)
	CalledProcessError: Command '['latex', '-halt-on-errror', 'c:\\users\\jorgenst\\appdata\\local\\temp\\tmpqf0hbq\\tmp.tex']' returned non-zero exit status 1

	======================================================================
	ERROR: Test that latex_to_png_dvipng just runs without error.
	----------------------------------------------------------------------
	Traceback (most recent call last):
	  File "c:\python27\lib\site-packages\nose\case.py", line 197, in runTest
	    self.test(*self.arg)
	  File "c:\python27\lib\site-packages\ipython-0.14.dev-py2.7.egg\IPython\lib\latextools.py", line 137, in latex_to_png_dvipng
	    stdout=subprocess.PIPE, stderr=subprocess.PIPE)
	  File "c:\python27\lib\subprocess.py", line 504, in check_call
	    raise CalledProcessError(retcode, cmd)
	CalledProcessError: Command '['latex', '-halt-on-errror', 'c:\\users\\jorgenst\\appdata\\local\\temp\\tmpczi2ji\\tmp.tex']' returned non-zero exit status 1

	======================================================================
	ERROR: Test that latex_to_png_dvipng just runs without error.
	----------------------------------------------------------------------
	Traceback (most recent call last):
	  File "c:\python27\lib\site-packages\nose\case.py", line 197, in runTest
	    self.test(*self.arg)
	  File "c:\python27\lib\site-packages\ipython-0.14.dev-py2.7.egg\IPython\lib\latextools.py", line 137, in latex_to_png_dvipng
	    stdout=subprocess.PIPE, stderr=subprocess.PIPE)
	  File "c:\python27\lib\subprocess.py", line 504, in check_call
	    raise CalledProcessError(retcode, cmd)
	CalledProcessError: Command '['latex', '-halt-on-errror', 'c:\\users\\jorgenst\\appdata\\local\\temp\\tmpvmepft\\tmp.tex']' returned non-zero exit status 1

	----------------------------------------------------------------------
	Ran 49 tests in 0.610s

	FAILED (errors=4)