tox: Use absolute path for -rrequirements.txt.

This fixes a FileNotFoundError on Ubuntu Trusty.

tox.ini

@@ -52,7 +52,7 @@ commands =
 skip_install = true
 setenv = PYTHONPATH={toxinidir}/scripts
 deps =
-    -rrequirements.txt
+    -r{toxinidir}/requirements.txt
     astroid==1.3.6
     beautifulsoup4==4.3.2
     pylint==1.4.3
@@ -76,7 +76,7 @@ commands = {envpython} -m pep257 scripts tests qutebrowser --ignore=D102,D103,D2
 [testenv:flake8]
 skip_install = true
 deps =
-    -rrequirements.txt
+    -r{toxinidir}/requirements.txt
     pyflakes==0.8.1
     pep8==1.5.7  # rq.filter: <1.6.0
     flake8==2.4.0
@@ -105,7 +105,7 @@ commands =
 skip_install = true
 whitelist_externals = git
 deps =
-    -rrequirements.txt
+    -r{toxinidir}/requirements.txt
 commands =
     {[testenv:mkvenv]commands}
     {envpython} scripts/src2asciidoc.py
@@ -115,7 +115,7 @@ commands =
 [testenv:smoke]
 passenv = DISPLAY XAUTHORITY HOME
 deps =
-    -rrequirements.txt
+    -r{toxinidir}/requirements.txt
 # We don't use {[testenv:mkvenv]commands} here because that seems to be broken
 # on Ubuntu Trusty.
 commands =