Recommend clang >= 6 as current code triggers a bug in clang 5

.cirrus.yml

@@ -22,7 +22,7 @@ linux_task:
     matrix:
         - name: linux_clang
           container:
-              image: silkeh/clang:5
+              image: silkeh/clang:6
           clang_script:
               - apt-get -y install pkg-config libncursesw5-dev
           env:

README.asciidoc

@@ -105,7 +105,7 @@ Building
 
 Kakoune dependencies are:
 
- * A {cpp}17 compliant compiler (GCC >= 7 or clang >= 5) along with its
+ * A {cpp}17 compliant compiler (GCC >= 7 or clang >= 6) along with its
    associated {cpp} standard library (libstdc{pp} or libc{pp})
  * ncurses with wide-characters support (>= 5.3, generally referred to as libncursesw)