Change /bin/bash to /bin/sh in tox.ini

No features of Bash are used here, so using /bin/sh makes it more portable to systems that don't have Bash natively (like BSD systems).

tox.ini

@@ -11,7 +11,7 @@ deps =
 setenv =
     PYTHONDONTWRITEBYTECODE = no_byte_code
 commands =
-    /bin/bash -c "find {toxinidir} -name '*.pyc' -delete ; coverage run {env:COVERAGE_OPTS:} --source={toxinidir}/synapse \
+    /bin/sh -c "find {toxinidir} -name '*.pyc' -delete ; coverage run {env:COVERAGE_OPTS:} --source={toxinidir}/synapse \
         {envbindir}/trial {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}"
     {env:DUMP_COVERAGE_COMMAND:coverage report -m}
 
@@ -26,4 +26,4 @@ skip_install = True
 basepython = python2.7
 deps =
     flake8
-commands = /bin/bash -c "flake8 synapse tests {env:PEP8SUFFIX:}"
+commands = /bin/sh -c "flake8 synapse tests {env:PEP8SUFFIX:}"