Fix Travis Perl versions

The Perl versions available under Travis have changed and many older
releases were removed. Make perlbrew build releases that aren't
installed. Reduce the number of tested versions, but add test with
blead.
diff --git a/.travis.yml b/.travis.yml
index 052e274..cb804d8 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,13 +21,10 @@
 env:
   - LUCY_DEBUG=1 CLOWNFISH_HOST=c
   - LUCY_DEBUG=1 CLOWNFISH_HOST=go
-  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.20
-  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.18
-  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.16
+  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=blead
+  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.24-extras
   - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.14
-  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.12
-  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.10
-  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.8
+  - LUCY_DEBUG=1 CLOWNFISH_HOST=perl PERL_VERSION=5.8.3
 git:
   depth: 10
 
diff --git a/devel/bin/travis-test.sh b/devel/bin/travis-test.sh
index 0927a29..d1a05cd 100755
--- a/devel/bin/travis-test.sh
+++ b/devel/bin/travis-test.sh
@@ -49,7 +49,8 @@
     export PERL_USE_UNSAFE_INC=0
 
     source ~/perl5/perlbrew/etc/bashrc
-    perlbrew switch $PERL_VERSION
+    perlbrew switch $PERL_VERSION ||
+        perlbrew install --switch --notest --noman --thread $PERL_VERSION
     perlbrew list
     export PERL5LIB="$install_dir/lib/perl5"