simplify test for contexts (is now by default an empty list)

repo2docker/buildpacks/r.py

@@ -239,19 +239,18 @@ class RBuildPack(PythonBuildPack):
             ),
         ]
 
-        if self.stencila_manifest_contexts:
-            if "r" in self.stencila_manifest_contexts:
-                scripts += [
-                (
-                    "${NB_USER}",
-                    # Install and register stencila library
-                    r"""
-                    R --quiet -e "source('https://bioconductor.org/biocLite.R'); biocLite('graph')" && \
-                    R --quiet -e "devtools::install_github('stencila/r', ref = '361bbf560f3f0561a8612349bca66cd8978f4f24')" && \
-                    R --quiet -e "stencila::register()"
-                    """
-                ),
-            ]
+        if "r" in self.stencila_contexts:
+            scripts += [
+            (
+                "${NB_USER}",
+                # Install and register stencila library
+                r"""
+                R --quiet -e "source('https://bioconductor.org/biocLite.R'); biocLite('graph')" && \
+                R --quiet -e "devtools::install_github('stencila/r', ref = '361bbf560f3f0561a8612349bca66cd8978f4f24')" && \
+                R --quiet -e "stencila::register()"
+                """
+            ),
+        ]
 
         return super().get_build_scripts() + scripts