stop installing stop installing qiskit-terra from source

This changes installation of qiskit-terra in tox.
Recent commit of Qiskit#1448 changed installation of qiskit-terra to use pypi in most CI workflow, but not tox.

tox.ini

@@ -11,7 +11,7 @@ setenv =
   LC_ALL=en_US.utf-8
 deps =
   -r requirements-dev.txt
-  git+https://github.com/Qiskit/qiskit-terra.git
+  qiskit-terra
 commands =
     python setup.py bdist_wheel -- -- -j4
     pip install --find-links={toxinidir}/dist qiskit_aer
@@ -31,15 +31,15 @@ commands =
 deps =
   pycodestyle
   pylint
-  git+https://github.com/Qiskit/qiskit-terra.git
+  qiskit-terra
 commands =
   pycodestyle --ignore=E402,W504 --max-line-length=100 qiskit/providers/aer
   pylint -j 2 -rn qiskit/providers/aer
 
 [testenv:docs]
 deps =
   -r requirements-dev.txt
-  git+https://github.com/Qiskit/qiskit-terra.git
+  qiskit-terra
   qiskit-ibmq-provider
 commands =
   python setup.py bdist_wheel -- -- -j4