Ensure that theano tests are not run under py.test if theano is not installed

* On master (without theano installed):

```bash
$ py.test sympy/printing
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.20 -- pytest-2.5.2
architecture: 64-bit
cache:        yes
ground types: python 

collected 419 items 

sympy/printing/pretty/tests/test_pretty.py .................................................................
sympy/printing/tests/test_ccode.py .........................
sympy/printing/tests/test_codeprinter.py ...
sympy/printing/tests/test_conventions.py ..
sympy/printing/tests/test_dot.py .........
sympy/printing/tests/test_fcode.py ................................
sympy/printing/tests/test_gtk.py x.
sympy/printing/tests/test_jscode.py ......................
sympy/printing/tests/test_lambdarepr.py ....
sympy/printing/tests/test_latex.py .....x.........................................................................x.....
sympy/printing/tests/test_mathematica.py .........
sympy/printing/tests/test_mathml.py .................
sympy/printing/tests/test_precedence.py ............
sympy/printing/tests/test_python.py .....x....
sympy/printing/tests/test_repr.py ....................
sympy/printing/tests/test_str.py ........................................................................
sympy/printing/tests/test_tableform.py ..
sympy/printing/tests/test_theanocode.py FFFFFFFFFF.F.FFFFFFFFFF.FFFE
```

* PR
```bash
$ py.test sympy/printing
============================= test session starts ==============================
platform linux -- Python 3.4.1 -- py-1.4.20 -- pytest-2.5.2
architecture: 64-bit
cache:        yes
ground types: python 

collected 419 items 

sympy/printing/pretty/tests/test_pretty.py .................................................................
sympy/printing/tests/test_ccode.py .........................
sympy/printing/tests/test_codeprinter.py ...
sympy/printing/tests/test_conventions.py ..
sympy/printing/tests/test_dot.py .........
sympy/printing/tests/test_fcode.py ................................
sympy/printing/tests/test_gtk.py x.
sympy/printing/tests/test_jscode.py ......................
sympy/printing/tests/test_lambdarepr.py ....
sympy/printing/tests/test_latex.py .....x.........................................................................x.....
sympy/printing/tests/test_mathematica.py .........
sympy/printing/tests/test_mathml.py .................
sympy/printing/tests/test_precedence.py ............
sympy/printing/tests/test_python.py .....x....
sympy/printing/tests/test_repr.py ....................
sympy/printing/tests/test_str.py ........................................................................
sympy/printing/tests/test_tableform.py ..
sympy/printing/tests/test_theanocode.py ssssssssssssssssssssssssssss

============== 387 passed, 28 skipped, 4 xfailed in 4.71 seconds ===============
```