setup: bump WebOb minimum version to 1.8

b075693b3214 introduced use of acceptable_offers which with WebOb < 1.8 would
fail with:

  File ".../kallithea/lib/middleware/pygrack.py", line 189, in __call__
    elif req.accept.acceptable_offers(self.valid_accepts):
AttributeError: 'MIMEAccept' object has no attribute 'acceptable_offers'

setup.py

@@ -43,7 +43,7 b' requirements = ['
     "alembic >= 0.8.0, < 1.1",
     "gearbox >= 0.1.0, < 1",
     "waitress >= 0.8.8, < 1.4",
-    "WebOb >= 1.7, < 1.9",
+    "WebOb >= 1.8, < 1.9",
     "backlash >= 0.1.2, < 1",
     "TurboGears2 >= 2.3.10, < 2.5",
     "tgext.routes >= 0.2.0, < 1",