youtube-dl: Update to 2018.03.20

And make python36 the default variant.

net/youtube-dl/Portfile

@@ -3,7 +3,7 @@
 PortSystem          1.0
 
 name                youtube-dl
-version             2018.03.10
+version             2018.03.20
 categories          net
 platforms           darwin
 maintainers         {ryandesign @ryandesign} openmaintainer
@@ -17,9 +17,9 @@ long_description    ${name} is a small ${description}.
 homepage            https://rg3.github.io/youtube-dl/
 master_sites        https://yt-dl.org/downloads/${version}
 
-checksums           rmd160  fc093953618d567b6d20bffab769e82e85308d5c \
-                    sha256  4bfadccb19e379ce38f5601c72dacf0ac5e03881230afee6df2152ab42fa75c5 \
-                    size    2925069
+checksums           rmd160  168d8c2edcfec616eaf1bb4744635d50e2a7ea63 \
+                    sha256  a977d04aa201ef35f8a6dc66b19204a9093b26c9d04d643089a1d467ba4e0682 \
+                    size    2934486
 
 depends_build       bin:zip:zip
 
@@ -93,7 +93,7 @@ variant python36 conflicts python27 python34 python35 description {Use Python 3.
 }
 
 if {![variant_isset python27] && ![variant_isset python34] && ![variant_isset python35] && ![variant_isset python36]} {
-    default_variants +python27
+    default_variants +python36
 }
 
 livecheck.type      regex