build: drop support for go 1.9; test with 1.14 and 1.15

Author: olt

.travis.yml

@@ -4,10 +4,10 @@ sudo: false
 language: go
 
 go:
-  - "1.9.x"
   - "1.10.x"
-  - "1.12.x"
-  - "1.13.x" # we use this for releases, see deploy.on.condition below
+  - "1.13.x"
+  - "1.14.x"
+  - "1.15.x" # we use this for releases, see deploy.on.condition below
   - tip
 
 matrix:
@@ -132,5 +132,5 @@ deploy:
   on:
     repo: omniscale/imposm3
     tags: true
-    condition: $TRAVIS_GO_VERSION =~ ^1\.13
+    condition: $TRAVIS_GO_VERSION =~ ^1\.15
 

README.md

@@ -103,7 +103,7 @@ There are some dependencies:
 
 #### Compiler
 
-You need [Go >=1.9](http://golang.org).
+You need [Go >=1.10](http://golang.org).
 
 #### C/C++ libraries