removed unnecessary restriction on minmatchLength

it's now transparently translated to nearest value when unsupported
(7->6) (3->4)

doc/zstd_manual.html

@@ -1,10 +1,10 @@
 <html>
 <head>
 <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
-<title>zstd 1.1.4 Manual</title>
+<title>zstd 1.1.5 Manual</title>
 </head>
 <body>
-<h1>zstd 1.1.4 Manual</h1>
+<h1>zstd 1.1.5 Manual</h1>
 <hr>
 <a name="Contents"></a><h2>Contents</h2>
 <ol>