Fix dummy inspection command

Change dummy inspection /usr/bin/true to true and hope that
the executing shell has it on the path, to fix travis error:
  In-toto verification for
  '[stripped]/final-product_0.0.0.0-0_all.deb' failed, reason was:
   [Errno 2] No such file or directory: u'/usr/bin/true'

See https://github.com/in-toto/in-toto/issues/247 for discussion
about dummy inspections.

tests/data/root.layout

@@ -2,8 +2,8 @@
  "signatures": [
   {
    "keyid": "88876a89e3d4698f83d3db0e72e33ca3e0e04e46",
-   "other_headers": "04000108001d16210488876a89e3d4698f83d3db0e72e33ca3e0e04e4605025c095e98",
-   "signature": "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"
+   "other_headers": "04000108001d16210488876a89e3d4698f83d3db0e72e33ca3e0e04e4605025c2e26cd",
+   "signature": "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"
   }
  ],
  "signed": {
@@ -29,7 +29,7 @@
     "expected_products": [],
     "name": "verify-reprobuilds",
     "run": [
-     "/usr/bin/true"
+     "true"
     ]
    }
   ],