upgrade typed-ast to fix python3.8 compiler error

```
-I/opt/python/3.8.0/include/python3.8 -c ast3/Python/ast.c -o build/temp.linux-x86_64-3.8/ast3/Python/ast.o
  ast3/Python/ast.c:22:1: error: conflicting types for ‘_PyObject_FastCall’
   _PyObject_FastCall(PyObject *func, PyObject *const *args, int nargs)
   ^
  In file included from /opt/python/3.8.0/include/python3.8/abstract.h:837:0,
                   from /opt/python/3.8.0/include/python3.8/Python.h:147,
                   from ast3/Python/ast.c:6:
  /opt/python/3.8.0/include/python3.8/cpython/abstract.h:145:1: note: previous definition of ‘_PyObject_FastCall’ was here
   _PyObject_FastCall(PyObject *func, PyObject *const *args, Py_ssize_t nargs)
   ^
  error: command 'gcc' failed with exit status 1
  ----------------------------------------
  ERROR: Failed building wheel for typed-ast
```
https://github.com/fphammerle/freesurfer-surface/pull/14/checks?check_run_id=441918682
https://travis-ci.org/fphammerle/freesurfer-surface/jobs/649600925?utm_medium=notification&utm_source=github_status

Pipfile

@@ -12,6 +12,8 @@ pytest = "<5"
 pytest-cov = "<3,>=2"
 "autopep8" = "<2"
 isort = "<5"
+# >=1.4.0 python3.8
+typed-ast = ">=1.4.0"
 
 [requires]
 python_version = "3"