Stop supporting Go older than 1.9 (#399)

Fixes #398.

.travis.yml

@@ -4,21 +4,19 @@ go_import_path: github.com/google/pprof
 
 matrix:
   include:
-    - os: linux
-      go: 1.7.x
-    - os: linux
-      go: 1.8.x
     - os: linux
       go: 1.9.x
+    - os: linux
+      go: 1.10.x
     - os: linux
       go: master
     - os: osx
       osx_image: xcode6.4
-      go: 1.8.x
+      go: 1.9.x
       env: SKIP_GRAPHVIZ=1
     - os: osx
       osx_image: xcode6.4
-      go: 1.9.x
+      go: 1.10.x
       env: SKIP_GRAPHVIZ=1
     - os: osx
       osx_image: xcode6.4
@@ -26,19 +24,19 @@ matrix:
       env: SKIP_GRAPHVIZ=1
     - os: osx
       osx_image: xcode7.3
-      go: 1.8.x
+      go: 1.9.x
     - os: osx
       osx_image: xcode7.3
-      go: 1.9.x
+      go: 1.10.x
     - os: osx
       osx_image: xcode7.3
       go: master
     - os: osx
       osx_image: xcode8.3
-      go: 1.8.x
+      go: 1.9.x
     - os: osx
       osx_image: xcode8.3
-      go: 1.9.x
+      go: 1.10.x
     - os: osx
       osx_image: xcode8.3
       go: master

README.md

@@ -27,7 +27,7 @@ them through the use of the native binutils tools (addr2line and nm).
 
 Prerequisites:
 
-- Go development kit. Requires Go 1.7 or newer.
+- Go development kit. Requires Go 1.9 or newer.
   Follow [these instructions](http://golang.org/doc/code.html) to install the 
   go tool and set up GOPATH.